Leidenfrost液滴之二元碰撞高效混合

Abstract

本研究利用一具有向心鋸齒狀表面之加熱表面，使兩顆半徑為1 mm至2 mm不等之Leidenfrost液滴在中心碰撞，促進二元混合，結果發現在mm尺度下的混合最短能在99.57 ms秒內完成，大幅提升混合效率。Leidenfrost液滴碰撞混合的過程可分為三個階段，階段I由液滴動量主導，其混合效果隨著撞擊速度增加或融合前接觸時間減少而增加，時間長短由撞擊速度決定，撞擊速度增加時，對水液滴影響不大，對乙醇液滴則會造成接觸面積增加，使階段I經歷的時間上升。階段II的混合主要依靠內流場之對流，混合效果隨著渦流產生及內流場平均流速升高而增加，其時間長短則與液滴抵抗變形的能力有關，水的表面張力較大，抵抗變形能力較佳，故在體積相同的情況下，水液滴的階段II皆較短，在撞擊速度越快的情況下，液滴亦需要越多時間使振幅趨於穩定，液滴體積上升使得蒸發量上升，蒸氣層流動速度變快而增加撞擊速度，故體積越大則階段II越長。階段III則由液滴震盪主導，混合效果隨著震盪頻率升高而增加，其時間長短則與表面震盪所造成之擾動有關，同時受到震盪頻率及體積的大小影響，震盪頻率降低與體積增加都會使表面震盪時對流場產生的擾動較小，混合將越慢完成。研究成果指出，液滴體積相同時，水液滴的混合效果皆比乙醇液滴好；當流體相同時，液滴體積越小則混合效果越好；當水與乙醇液滴碰撞混合時，由於兩液滴的飽和溫度不同，融合後乙醇被水加熱，使得內部產生氣泡，內流場的熱對流及氣泡對流場的攪動促使混合效果變好，然而當水液滴的體積增加為乙醇液滴的3倍時，液滴內發生核沸騰，氣泡持續增大與破裂將影響內流場對流，致使混合效果變差。

This study focused on the mixing performance of two Leidenfrost drops with a radius of 1 to 2 mm. Two drops were deposited on the opposite edge of a superheated ratchet surface and propelled to collide at the center. Experimental results indicated that, best mixing can be achieved within 99.57 ms. The process of mixing and coalescence of Leidenfrost drops could be divided into 3 stages. Stage I was dominated by the momentum of the two head-on drops, so that mixing was enhanced by the increase of the approaching velocity. For ethanol drops, higher approaching velocity also led to a longer contact time. This resulted in longer stage I and less effective mixing. In stage II, convection became more important to the mixing process. Vortices were generated inside the merged drop by the shear stress of vapor film. Due to the faster inner flow, water drops always outperformed ethanol drops in mixing when their volume were identical. Stage III was dominated by surface oscillation of the coalesced mass. The elapsed time of stage III depended on the oscillation frequency and the volume of the merged drop. When water drop collided to ethanol drop, temperature difference existed within the merged drop. As a result, ethanol vaporized to produce bubble nuclei. Isolated bubbles were observed when the volume ratio of water to ethanol was less than 3. The agitation caused by the tiny bubbles increased the mixing efficiency. However, the intense bubble growth when the volume of water drop was three times larger than ethanol drop was found to mitigate the mixing process.